1. Field of the Invention
This present invention relates, in general, to repairing or sealing leaking elongate hollow members such as control rod drive housings and in-core monitor housings in a reactor pressure vessel of a nuclear reactor such as a boiling water reactor.
2. Description of the Related Art
Boiling water nuclear reactors typically may include a reactor core located within a reactor pressure vessel (RPV). A known RPV may include a substantially cylindrical shell. The shell, for example, can be about twenty feet in diameter and about seven inches thick.
The cylindrical shell may be closed at its top end by a removable top head. The top head may be removable so that components, such a fuel bundles, located in the RPV can be accessed. The RPV cylindrical shell may be closed at its bottom end by a dome shaped bottom head assembly welded to the shell.
A plurality of openings may be formed in the bottom head dome so that components, such as control rod drive assemblies, can extend within the RPV. Typically, a substantially cylindrical stub tube having a bore extending there through may be welded to the bottom head dome so that the tube bore aligns with an opening in the bottom head dome. The cylindrical stub tube may typically be fabricated from a corrosion resistant material such as stainless steel or Ni—Cr—Fe.
As an example, for a control rod drive assembly, the control rod drive housing, e.g., a tube, may be inserted through the bottom head dome opening and stub tube bore, and the housing may extend into the RPV. The control rod drive (CRD) housing may be welded to the stub tube to maintain the housing in a desired position. The stub tube thus may serve as a transition piece between the bottom head dome, which typically may be fabricated from low alloy steel, and the CRD housing, which typically may be fabricated from stainless steel such as 304 stainless steel with a high carbon content.
Inter-granular stress corrosion cracking (IGSCC) is a known phenomenon occurring adjacent to stub tube welds connecting the bottom head dome to the stub tube and connecting the stub tube to the CRD housing. Particularly, the stub tube welds may be subject to a variety of stresses associated with, for example, differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stresses from welding, cold working and other inhomogeneous metal treatments. Such stresses may, at times, cause cracks adjacent the stub tube welds.
If stress corrosion cracks adjacent stub tube welds are not sealed, such cracks may cause potential leakage paths between the stub tube and the bottom head dome, and the stub tube and the CRD housing, respectively, which is undesirable. Accordingly, upon detection of any such cracks, it may be desirable to re-seal the control rod drive housing, for example, to the bottom head dome.
Type 304 stainless steel stub tubes in some plants have become furnace sensitized as a result of vessel post weld heat treatment. This has left the stub tube in an inter-granular stress corrosion cracking (IGSCC) susceptible state, and has led to leaking cracks. Cracking has been observed in the heat affected zone of the stub tube at the CRD housing attachment weld of this susceptible material. This results in a reactor coolant leakage path to the under-vessel area. Restoration of the defective area is virtually impossible due to the location of the stub tubes and the existing material condition.
One known method of repairing or re-sealing CRD housings within the bottom head dome includes completely replacing the stub tube and CRD housing. This method, however, is time consuming, tedious, and expensive. Particularly, the housing and associated stub tube are partially cut-off and removed from the bottom head dome, and the material remaining in the bottom head assembly is inspected to ensure that such material can be welded without damaging the bottom head. A weld build-up may then be formed over the remaining material and machined so that a new stub tube can be welded to the weld build-up. Several weeks can be required to perform the replacement process of just one penetration tube. Moreover, much of the replacement work must be performed within the RPV, which requires completely unloading the RPV and is undesirable.
Another known method of repairing or re-sealing a CRD housing within the bottom head dome includes welding a sleeve to the CRD housing and the stub. This method, however, only addresses stress corrosion cracks adjacent the interface between the stub tube and the CRD housing. Moreover, installing the sleeve must be performed entirely within the RPV which, as explained above, is undesirable.
Another known method of repairing or re-sealing a CRD housing within the bottom head dome includes rolling the CRD housing into the bottom head dome. While this method may be quicker than replacing the stub tube and CRD housing, rolling the CRD housing into the bottom head dome does not create as tight a seal as a weld between the CRD housing and the dome. Moreover, the rolled CRD housing may become separated from the bottom head dome after continued RPV operation, and must then be re-rolled. Re-rolling a CRD housing, however, often is neither desirable nor practical.
Another known method of repairing or re-sealing a CRD housing within the bottom head dome includes removing and replacing a lower portion of the CRD housing within the bottom head dome. Particularly, a lower portion of the CRD housing is cut-off and removed so that an upper portion of the CRD housing remains inserted in an opening in the bottom head dome and welded to the stub tube. The bottom head dome is then cleaned, and the lower end of the remaining CRD housing upper portion is machined so that a replacement bottom portion of CRD housing can be welded to the remaining upper portion.
The replacement bottom portion of CRD housing is similarly machined so that it can be welded to the remaining upper portion. The replacement bottom portion of CRD housing is then inserted into the bottom head dome opening and positioned beneath the remaining upper portion. The replacement bottom portion and the remaining upper portion of CRD housing are temper bead welded to each other and to the bottom head dome.
Temper bead welding the remaining upper portion to the replacement lower portion and the bottom head dome may have the undesirable effect of causing high stresses because of thermal growth mismatch between the CRD housing and the bottom head dome, which are fabricated from different materials. Such temper bead welding also has the undesirable potential effect of trapping water within a leakage path and in contact with the weld between the remaining upper portion and the bottom head dome.